Water cooled crucible for high frequency heating



Oct. 13, 1959 T. RUMMEL 2,908,739 I WATER COQLED CRUCIBLE FOR HIGHFREQUENCY HEATING Filed May 15, 1957 2 Sheets-Sheet 1 oil 12 9a 9 1 Ijzuew r.

Reactor fummgl? Oct. 13, 1959 T. RUMMEL 2,908,739

WATER COOLED CRUCIBLE FOR HIGH FREQUENCY HEATING Filed May 15, 1957 2Sheets-Sheet 2 Inventor. I jxaalorfiwmmefi United States Patent WATERCOOLED CRUCIBLE FOR rnon FREQUENCY HEATING Theodor Rummel, Munich,Germany, assignor to Siemens & Halske Aktiengesellschaft, Berlin andMunich, Germany, a German corporation The present invention relates to adevice comprising a metallic crucible, particularly for use in melting,by high-frequency induction heating, highly pure semi-conductivematerials, forinstance, silicon or germanium to be employed, forexample, in the production of therrnistors, transistors, fieldistors,diodes and the like.

crucibles which are heated by high frequency are broadly known. Priorembodiments have, however, in general a poor efliciency, that is, onlypart of the energy given off by the primary coil of the high frequencydevice is utilized for heating the substance to be melted. The cruciblesmust therefore be made of a material which has a substantially highermelting point than the material to be melted and which does not reactwith the material to be melted.

The object of the present invention is to provide, for the aboveindicated purpose, a device including a crucible in which the highfrequency energy is utilized with high efficiency by closely couplingthe primary coil with the crucible and in which furthermore, there canbe used as crucible material even metallic substances which have a lowermelting point than the material to be melted and/ or could react withthe material to be melted without contamination resulting therefrom bythe dissolving of the crucible material in the melt or reaction with it.

In accordance with the invention, this object is achieved by theprovision of a cooled secondary coil which is particularly closelycoupled with the primary coil and forms in whole or in part a cruciblecomprising a heat-resistant insulating bottom which is preferablyoutside the action of the high frequency field. The invention alsocontemplates a method of melting semi-conductive substances in meltingdevices made in accordance therewith.

Part of the secondary coil may for instance directly form the side wallof a crucible. This may be obtained, for example, by winding a tube ofrectangular crosssection so as to form a cylindrical coil traversed by asuitable coolant, for instance, water, and having an insulating membermade of heat resistant material, for example, quartz glass or a ceramicmaterial, forming the bottom thereof. The ends of the cylinder coil areconnected with another part of the secondary coil, likewise traversed bythe coolant, and cooperatively disposed with respect to the primaryhigh-frequency coil. The spacings between the water-cooled turns of thecrucible coil are so narrow that the material to be melted cannot escapetherethrough. It is furthermore advisable to provide the secondary coilwith substantially fewer number of turns than the primary coil so as topass through the material to be melted the highest possible current.

One very advantageous further feature of the invention resides informing the crucible as a part'of a cooled secondary coil comprisingonly a single turn, in such a manner that two hollow bodies, inparticular of cylindrical shape and having double walls and open at eachend which are slit open along a generatrix, are conductively connectedalong the common slit by the wall of one of the hollow bodies which isdisposed adjacent the 2,908,739 Patented Oct. 13, 1959 primary coil,such wall passing into the inner wall of the second hollow body formingthe crucible and provided with an insulating heat-resistant bottomdisposed preferably outside of the high-frequency field, producing inthis way a closed current path.

For the melting of semiconductor compounds, in melting devices made inaccordance with the invention, it is not advantageous in all cases toconnect the high-frequency heating at the very start of the meltingoperation since the material to be melted has a considerable resistancein cold state and therefore high frequency energy is taken up by it onlywith very low efiiciency. For this reason, the material to be melted isinitially suitably heated by means of a gas discharge, or electricdischarge, for instance with high cathode potential drop, or by an arcdischarge or the like, until the conductivity thereof has becomesufliciently high so that the heating may be continued with highefliciency by high-frequency until the molten state is obtained.

The various objects and features of the invention will appear from thedescription, which is rendered below, with reference to the accompanyingdrawing, wherein Fig. 1 shows one embodiment in sectional view;

Fig. 1a shows the structure of Fig. l in simplified perspective view;

Fig. 2 is a vertical sectional view of another embodiment;

Fig. 3 shows a transverse sectional view of the embodiment illustratedin Fig. 2; and

Fig. 4 illustrates in sectional view an embodiment comprising a cruciblemade of cross-sectionally rectangular tubing wound to form a cylindricalcoil.

Referring now to Figs. 1 and la, the primary coil 3 of thehigh-frequency device, having two terminals 3 and 3", is disposed withina hollow cylinder 1 having the walls 1' and 1" slightly spaced therefromas indicated in Fig. 1 at 7. The crucible 2, forming the melting chamber4, is disposed outside this cylinder. It has two walls 2' and 2" and isconductively connected with the cylinder 1 by Way of walls 6', 6" and6a, 6b forming channels which communicate with the channels 1 and 2 andalso forming a central channel 5 extending from the crucible 2 to thecylinder 1. The channels 1 and 2 are traversed by a coolant, preferablywater. The high frequency energy which is suitably fed to the primarycoil 3 and which should have a high voltage and low amperage, istransmitted with only slight energy losses at low voltage and highamperage to the inner wall 1" and from there over the connecting walls 6and 6" to the inner wall 2" of the crucible. The primary coil 3 may forinstance have 30 turns so that the voltage in the secondary coil isstepped down to about one 30th of the voltage in the primary coil, butthe current becomes about 30 times as high. In order to increase thecurrent density within the crucible, it is furthermore advisable to keepits crosssection small by making the crucible substantially shorter thanthe cylinder 1 disposed adjacent to the primary coil 3. As indicated inFig. la by numeral 16, the wall of the crucible 2 may be lined by alayer of powdered material corresponding to the material to be melted.

As is particularly shown in Fig. 1b, electrodes such as E and E may bedisposed directly above the material 4 which is to be melted, suchelectrodes being connected to a voltage source U for producing adischarge serving for initially preheating the semiconductor material 4so as to increase its conductivity to a value at which thehigh-frequency energy supplied by the source HF and given off from thehigh-frequency primary coil 3 to the cylinder 1 and therewith to thesecondary coil 2 (crucible) and to the material 4, can be eflicientlyemployed to continue heating the material to melt it. Numeral 15 indicates the bottom of the crucible 2, which is made of heatr. c 3resistant material, and 15', 15" indicate connections for the coolingmedium.

In Figs. 2 and 3, numeral 3 is the high-frequency primary coil havingthe terminals 3' and 3".' Numerals 9" and 9" are the wallsfof acylindrical secondary coil coupled with the high-frequency primary coil3 and forming an inner cavity 9, the walls 9' and 9" being conductivelyinterconnected over connecting walls 9a and 9b with the walls of anothercylindrical metallic member disposed coaxially therein and having thewalls 10 and 10" forming the cavity 10. The cavities 9 and 10 aretraversed by a coolant by way of the channels formed by the connectingwalls 9a and 9b. The inner walls such as 9a extending radially inwardlyand connecting with the inner wall of the cylindrical member forming thecavity 10 are separated by a slot 12. Numeral 13 indicates the spaceformed between the inner wall 9" of the outer secondary coil portion andthe outer wall 10' of the inner secondary coil portion forming thecrucible for melting the material 4. The inner chamber of the innercylinder having the. walls 10 and 10" is charged with the material to bemelted, for instance, semiconductor material 4. It is equipped with aninsulating bottom 15 in the form of a plate or a shallow dish (Fig. 2)which may consist for instance of quartz or ceramic and shouldpreferably lie outside the field. The inner wall of the inner cylinder10" gives off the high frequency energy to the material to be melted.The current path-of the high frequency on the secondary coil extendsessentially from the outer wall 9 of the outer cylinder by way of thewalls forming the slot 12 to the inner wall 10 of the inner cylinder.Numerals 15 and 15 indicate connections for the circulation of thecooling medium.

In order to obtain a high current density in the crucible, it isadvisable, as described in the previous embodiment, to make the crucible(10, 10', 10") shorter than the surrounding cylinder (9, 9', 9").

The advantage of the arrangement according to Figs. 2 and 3 is inparticular that in addition to a very close coupling between thehigh-frequency primary coil and the secondary coils, one of which formsthe crucible, with efliciencies of close to 1, it can also beconstructed in an extremely compact manner. This is advantageousparticularly when the melting is to be effected in a protective gasatmosphere.

Fig. 4 shows a structure wherein the side wall of the crucible is formedby a crossrsectionally rectangular tubing 21 wound to form a cylindricalbody provided with a bottom 15 made of heat resistant insulatingmaterial such as quartz glass or ceramic and the like. Numeral 4 againindicates the material to be melted. The ends of the tubing 21, forminga portion of the secondary coil, are conductively connected with anothercooperating secondary coil portion 2111 which is closely coupled withthe high frequency primary coil 3, and the latter is connected with thehigh-frequency source HF. Both portions ofthe secondary coils 21a and 21(the latter forming the crucible 20) are in operation traversed by acooling medium such as water, the inlet and outlet being indicated byarrows. The secondary coil portion 21a has advantageously a smallernumber of turns than the highfrequency primary coil 3 so as to providefor high current flow through the crucible 20. The turns of the tubing21 are closely adjacent so as to reliably prevent flow of moltenmaterial through the corresponding slot formed thereby.

The slot provided in all embodiments is so narrow that the moltenmaterial cannot flow out of the crucible. This is prevented inparticular by the high surface tension of the molten material. The.cooling may also be made so strong that the material to be melted andseeping into the slot is not heated to the melting point and thereforeremains thickly viscous or solid. Furthermore, the property of thesemiconductor material tobe melted, of being a poor conductor in-coldcondition, is favorable.

The current will preferably flow through the heated inner part of goodconductivity of the material to be melted and heat it, while the poorlyconductive cooled outer layers are passed through by a small amount ofthe current. It has been found by experience that the width of the slotmay be made about 0.5 to 2 mm.

For the cooled crucible there may also be used metallic substances whichhave a lower melting point than the material which is to be melted. Forinstance, copper or silver are very well suited for this purpose, butother metals of good conductivity such as aluminum may also be employed.

The wall thickness of the material to be used for making the cylindricalmembers may be about 0.5 to 2 mm.

The melting is preferably carried out ina gas protective atmosphere, forinstance in a hydrogen atmosphere.

Changes may be made within the scope and spirit of the appended claims.

I claim: v V

1. In apparatus forheat-treating highly pure semiconductor materialdeposited in a metallic crucible provided with means for cooling wallparts thereof, a device for inducing high-frequency current into saidmaterial deposited in said crucible to elfect melting thereof, saiddevice comprising a primary coil connected to a high frequency currentsource, a secondary coil closely coupled with said primary coil, andmeans for conductively interconnecting said metallic crucible with saidsecondary coil to form a closed circuit therewith for the flow ofhigh-frequencyenergy therethrough and through said material deposited insaid crucible.

2. A structure and cooperationof parts according to claim 1, comprisinga crucible having a narrow slot formed in a cooled wall part thereof,molten material initially seeping into said slot being cooled thereinand plugging said slot to prevent outflow of molten material from saidcrucible.

3. A structure and cooperation of parts according to claim 1, whereinsaid crucible comprises a bottom wall made of heat resistant insulatingmaterial and a metallic side wall extending from said bottom wall, saidside wall being hollow and being cooled by a coolant flowingtherethrough, a narrow slot being formed in said side wall, moltenmaterial initially seeping into said slot being cooled therein andplugging said slot to prevent outflow of molten material from saidcrucible.

4. A structure and cooperation of parts according to claim 1, comprisingmeans for producing an electric discharge to preheat said semiconductormaterial deposited in said crucible prior to applying high-frequencyenergy thereto which is inducedinto said material to eifect the meltingthereof.

5. A structure and cooperation of parts according to claim 1, comprisinga crucible having a slot formed therein, the width of said slot beingabout 0.5 to 2 mm., molten material initially seeping into said slotbeingcooled therein and plugging said slot to prevent outflow of moltenmaterial from said crucible.

6. A structure and cooperation of parts according to claim 1, whereinthe inner wall of said crucible is coated with a layer of powderconsisting of the material to be melted and about one-tenth to aboutone-hundredth millimeter thick. 7

7. A structure and cooperation of parts according to claim 1, wherein'said crucible is made of material of good metallic conductivity with awall thickness of about 0.5to2 mm. l e I 8. A structure and cooperationof parts according to claim 1, wherein said crucible comprises 'a'bottom wall made of heat resistant insulating material, a hollowmetallic member extending fromsaid bottom wallin the manner of a singleturn and formingthe side wall of said crucible, said hollow side wallbeing cooled by a coolant flowing therethrough, a narrow axiallyextending slot being formed in saidside wall, molten material in itiallyseeping into said slot being cooled therein and plugging said slot toprevent outflow of molten material from said crucible.

9. A structure and cooperation of parts according to claim 1, comprisinga secondary coil made of a doublewalled hollow member extending in themanner of a single turn which is closely coupled with said primary coil,said crucible comprising a bottom wall made of heat resistant insulatingmaterial, another double-walled hollow metallic member extending fromsaid bottom wall in the manner of a single turn and forming the sidewall of said crucible, wall means for metallically interconnecting therespective walls of said double-walled members to form a closed circuitfor the flow of high-frequency energy including said crucible and saidsecondary coil and also forming a narrow slot extending from the insideof said crucible to the inside of said secondary coil, molten materialinitially seeping into said slot being cooled therein and plugging saidslot to prevent outflow of molten material from said crucible.

10. A structure and cooperation of parts according to claim 9, whereinthe axial length of said secondary coil appreciably exceeds the axiallength of said crucible.

11. A structure and cooperation of parts according to claim 9, whereinsaid secondary coil is disposed radially spaced from said crucible.

12. A structure and cooperation of parts according to claim 9, whereinsaid crucible is disposed within said secondary coil coaxially thereof.

13. A structure and cooperation of parts according to claim 1, whereinsaid crucible comprises a wall formed of a generally spirally coiledmetallic tube, said tube being electrically conductively interconnectedwith said secondary coil and being cooled by a cooling medium flowingtherethrough, the turns of said coiled tube being mutually closelydisposed forming narrow slots therebetween, molten material initiallyseeping into said slots being cooled therein and plugging said slotsagainst outflow of molten material from said crucible.

14. A structure and cooperation of parts according to claim 1, whereinsaid crucible comprises a bottom wall made of heat resistant insulatingmaterial and a side wall extending from said bottom wall, said side wallbeing formed of a generally spirally coiled metallic tube, said tubebeing electrically conductively interconnected with said secondary coiland being cooled by a cooling medium flowing therethrough, the turns ofsaid coiled tube being mutually closely disposed forming narrow slotstherebetween, molten material initially seeping into said slots beingcooled therein and plugging said slots against outflow of moltenmaterial from said crucible.

15. A structure and cooperation of parts according to claim 1, whereinsaid crucible comprises a generally spirally coiled cross-sectionallyrectangilar metallic tube, said secondary coil being formed of a coiledtubing which is closely coupled with said primary coil and metallicallyinterconnected with said coiled tube forming said crucible, means forconducting a cooling medium for circulation through said secondary coiland through the coiled tube forming said crucible, the turns of saidcoiled tube being mutually closely disposed forming a continuous narrowslot, molten material initially seeping into said slot being cooledtherein and plugging said slot against outflow of molten material fromsaid crucible.

References Cited in the file of this patent UNITED STATES PATENTS760,057 Cowles May 17, 1904 1,378,187 Northrup May 17, 192.1 1,855,750Long Apr. 26, 1932 2,686,864 Wroughton et al. Aug. 17, 1954 2,693,498Penberthy Nov. 2, 1954

